Two plum cultivars (Pioneer and Celebration), grown in a private commercial orchard at El-Khatatba region under the Egyptian climaticconditions, were used to examine their self and cross compatibility and or incompatibility between both of them during the two successive experimental seasons 2017/2018 and 2018/2019. The fluorescence microscope was used to determine the degree of self and cross compatibility and or incompatibility, initial, and final fruit set percentage were calculated. Pollens of the two plum cultivars exhibited high rate of viability after 24 h in T.T.C. test and the germinated pollens of Pioneer cv. on Celebration style tissue were the highest. The combination between Celebration × Pioneer seemed to be as high partial cross compatible compared to the other treatments. The two cultivars under this study considered as partial self-compatible. Pioneer cv. had a higher degree of partial self-incompatibility than Celebration cv. Therefore, they need pollinizer cultivars for a good fruit set. Concerning the percentage of fruit set, the data revealed that fruiting of Pioneer and Celebration plum cultivars were good in the initial and final fruit set percentage in both seasons. Open pollination gave a significant difference in the initial and final fruit set percentages between the two plum cultivars followed by cross and self - pollination.